HttpClient using libcurl, Unix Sockets for HTTP. HTTPS support (#770)

The main goal of this change is to eliminate the cpr back-end altogether and replace it with the curl implementation. I would expect to drop cpr as soon as we feel happy with the libcurl back-end. That would leave us with a direct dependency on libcurl only, and cpr can be eliminated as a dependency.

### HttpClient Backend Overhaul
- Implemented a new **libcurl-based HttpClient** backend (`httpclientcurl.cpp`, ~2000 lines)
  as an alternative to the cpr-based one
- Made HttpClient backend **configurable at runtime** via constructor arguments
  and `-httpclient=...` CLI option (for zen, zenserver, and tests)
- Extended HttpClient test suite to cover multipart/content-range scenarios

### Unix Domain Socket Support
- Added Unix domain socket support to **httpasio** (server side)
- Added Unix domain socket support to **HttpClient**
- Added Unix domain socket support to **HttpWsClient** (WebSocket client)
- Templatized `HttpServerConnectionT<SocketType>` and `WsAsioConnectionT<SocketType>`
  to handle TCP, Unix, and SSL sockets uniformly via `if constexpr` dispatch

### HTTPS Support
- Added **preliminary HTTPS support to httpasio** (for Mac/Linux via OpenSSL)
- Added **basic HTTPS support for http.sys** (Windows)
- Implemented HTTPS test for httpasio
- Split `InitializeServer` into smaller sub-functions for http.sys

### Other Notable Changes
- Improved **zenhttp-test stability** with dynamic port allocation
- Enhanced port retry logic in http.sys (handles ERROR_ACCESS_DENIED)
- Fatal signal/exception handlers for backtrace generation in tests
- Added `zen bench http` subcommand to exercise network + HTTP client/server communication stack

1 files changed, 12 insertions, 0 deletions

diff --git a/src/zenhttp-test/zenhttp-test.cpp b/src/zenhttp-test/zenhttp-test.cpp
index b4b406ac8..0a6980462 100644
--- a/src/zenhttp-test/zenhttp-test.cpp
+++ b/src/zenhttp-test/zenhttp-test.cpp
@@ -1,6 +1,7 @@
 // Copyright Epic Games, Inc. All Rights Reserved.
 
 #include <zencore/testing.h>
+#include <zenhttp/httpclient.h>
 #include <zenhttp/zenhttp.h>
 
 #include <zencore/memory/newdelete.h>
@@ -9,6 +10,17 @@ int
 main([[maybe_unused]] int argc, [[maybe_unused]] char* argv[])
 {
 #if ZEN_WITH_TESTS
+	using namespace std::literals;
+	for (int i = 1; i < argc; ++i)
+	{
+		std::string_view Arg(argv[i]);
+		if (Arg.starts_with("--httpclient="sv))
+		{
+			std::string_view Value = Arg.substr(13);
+			zen::SetDefaultHttpClientBackend(Value);
+		}
+	}
+
 	return zen::testing::RunTestMain(argc, argv, "zenhttp-test", zen::zenhttp_forcelinktests);
 #else
 	return 0;